5822 | Chem. Commun., 2025, 61, 5822–5825 This journal is © The Royal Society of Chemistry 2025 Cite this: Chem. Commun., 2025, 61, 5822 Insight into the mechanism of semi- hydrogenation of phenylacetylene over Pd embedded in thioether functionalized Schiff-base linked covalent organic frameworks† Yiyong Zhao,‡ ab Jianyu Sun,‡ ab Jianxin Miao, ab Xinhui Zhang, ab Han Wu, ab Qunfeng Zhang, ab Yongwu Peng, a Chengrong Ding, a Jinghui Lyu * ab and Xiaonian Li ab We report a novel catalyst comprising palladium supported on thioether functionalized Schiff-base conjugated covalent organic frameworks, which significantly enhances the semi-hydrogenation conversion and selectivity of phenylacetylene. Theoretical calcula- tions substantiate that the imide and thioether groups modify the electron density around Pd, reducing the energy barriers for phe- nylacetylene adsorption and styrene desorption, thereby improving the conversion and selectivity of the catalyst. Semi-hydrogenations of alkynes to corresponding alkenes play a key role in the production of plastics, rubbers, resins, and pharmaceuticals. 1–3 In particular, the selective hydrogenation of phenylacetylene to styrene is one of the most commonly used methods for solving the puzzle of catalyst poisoning in styrene polymerization. 4–7 Noble-metal catalysts such as palladium, platinum, and gold exhibit excellent performance in selective hydrogenation reactions and are extensively employed in var- ious hydrogenation processes. 8–10 Although catalysts based on palladium are among the most effective in selective hydrogena- tion reaction due to their strong ability to dissociate hydrogen, they often struggle with timely alkene desorption, leading to over-hydrogenation. 11,12 Recent strategies to enhance the selec- tive hydrogenation performance of Pd-based catalysts include introducing a second metal (e.g., Ru/Pd or Zn/Pd) to alter the structural and electronic properties of the catalyst, 13,14 and using modifying agents such as quinoline, thiols, and other heteroatom-containing compounds to passivate the catalyst. 15,16 While both strategies aim to modulate the spatial and electronic environment surrounding Pd atoms to suppress catalytic activ- ity while enhancing selectivity, achieving an optimal balance between activity and selectivity in semi-hydrogenation of alkynes remains a significant challenge in the design of advanced catalyst architectures. 17–20 Since the synthesis of crystalline organic polymer, covalent organic frameworks (COFs), by Yaghi in 2005, have garnered significant attention in materials science, environmental science, and organic synthesis due to their high crystallinity, porosity, chemical tunability, and structural stability. 21–25 The morphology- controllable ability of COFs by adjusting different basic building blocks allows for diverse functionalities, while their regular porous structure and functional groups can effectively anchor metal active sites and prevent their aggregation. 26–29 Therefore, COFs have emerged as ideal materials for catalysis, offering a potential strategy to the challenge of balancing the selectivity and reactivity in the semi-hydrogenation of alkyne. Although various modifications to palladium have been proposed to boost the selectivity of semi- hydrogenation, most of the organic modifiers are toxic and tend to leach out during the reaction, leading to a decrease in catalyst selectivity. In 2018, Zheng’s group demonstrated that thiol treat- ment of palladium nanosheets enhances the selectivity of palladium catalysts. 30 Unlike thiol-based modification, using COFs with pre- designed sulfur-containing groups as supports for palladium cata- lysts effectively addresses the leaching problem of modifiers. In 2024, a vinylene-linked COF functionalized with thioether groups (–SCH 3 ) was reported to boost the semi-hydrogenation selectivity of alkynes. 31 Both experimental and theoretical calculation results demonstrated that the presence of –SCH 3 groups significantly improves the selectivity of palladium-based catalysts in comparison to vinylene-linked COFs without the –SCH 3 groups. Compared to vinylene-linked COFs, Schiff-base conjugated COFs provide the imine groups with more coordinative sites for metal atoms. 32 Hence, we designed a novel catalyst of palladium supported on Schiff-base linked COFs functionalized with –SCH 3 groups in accordance with the theory of metal–support interaction. The imide and thioether groups interact with Pd through sulfur and nitrogen atoms, allowing the Pd to be stabilized and immobilized in structure pores while the porous structure restricts the size of the a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, China. E-mail: lyujh@zjut.edu.cn b Zhejiang Key Laboratory of Surface and Interface Science and Engineering for Catalysts, Zhejiang University of Technology, Hangzhou, 310014, China † Electronic supplementary information (ESI) available: Experimental details, SEM, TEM, PXRD, etc. See DOI: https://doi.org/10.1039/d4cc06758d ‡ Y. Y. Zhao and J. Y. Sun contributed equally to this work. Received 27th December 2024, Accepted 17th March 2025 DOI: 10.1039/d4cc06758d rsc.li/chemcomm ChemComm COMMUNICATION Published on 18 March 2025. Downloaded by Zhejiang University of Technology on 8/13/2025 12:09:16 PM. View Article Online View Journal | View Issue